This invention relates to combustors for gas turbine engines and particularly to a combustor that reduces emissions of oxides of nitrogen (NOx) and whose exhaust gases have a prescribed spatial temperature profile.
Gas turbine engines, such as those used to power modern commercial aircraft, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. The combustor typically comprises radially spaced apart inner and outer liners. The liners define an annular combustion chamber that resides axially between the compressor and the turbine. Arrays of circumferentially distributed combustion air holes penetrate each liner at multiple axial locations to admit combustion air into the combustion chamber. A plurality of circumferentially distributed fuel injectors projects into the forward end of the combustion chamber to supply the fuel.
Combustion of the hydrocarbon fuel inevitably produces a number of pollutants including oxides of nitrogen (NOx). NOx emissions are the subject of increasingly stringent controls by regulatory authorities. Accordingly, engine manufacturers strive to minimize NOx emissions.
A principal strategy for minimizing NOx emissions is referred to as rich burn, quick quench, lean burn (RQL) combustion. The RQL strategy recognizes that the conditions for NOx formation are most favorable at elevated combustion flame temperatures, i.e. when the fuel-air ratio is at or near stoichiometric. A combustor configured for RQL combustion includes three serially arranged combustion zones: a rich burn zone at the forward end of the combustor, a quench or dilution zone axially aft of the rich burn zone, and a lean burn zone axially aft of the quench zone.
During engine operation, a portion of the pressurized air discharged from the compressor enters the rich burn zone of the combustion chamber. Concurrently, the fuel injectors introduce a stoichiometrically excessive quantity of fuel into the rich burn zone. The resulting stoichiometrically rich fuel-air mixture is ignited and burned to partially release the energy content of the fuel. The fuel rich character of the mixture inhibits NOx formation in the rich burn zone by suppressing the combustion flame temperature and also resists blowout of the combustion flame during any abrupt reduction in engine power.
The fuel rich combustion products generated in the rich burn zone then enter the quench zone where the combustion process continues. Jets of pressurized air from the compressor enter the combustion chamber radially through combustion air holes. The air mixes with the combustion products entering the quench zone to support further combustion and release additional energy from the fuel. The air also progressively deriches the fuel rich combustion products as they flow axially through the quench zone and mix with the air. Initially, the fuel-air ratio of the combustion products changes from fuel rich to stoichiometric, causing an attendant rise in the combustion flame temperature. Since the quantity of NOx produced in a given time interval increases exponentially with flame temperature, substantial quantities of NOx can be produced during the initial quench process. As the quenching continues, the fuel-air ratio of the combustion products changes from stoichiometric to fuel lean, causing an attendant reduction in the flame temperature. However, until the mixture is diluted to a fuel-air ratio substantially lower than stoichiometric, the flame temperature remains high enough to generate considerable quantities of NOx.
Finally, the deriched combustion products from the quench zone flow axially into the lean burn zone where the combustion process concludes. Additional jets of compressor discharge air are admitted radially into the lean burn zone. The additional air supports ongoing combustion to release energy from the fuel and regulates the peak temperature and spatial temperature profile of the combustion products. Regulation of the peak temperature and temperature profile protects the turbine from exposure to excessive temperatures and excessive temperature gradients.
Because most of the NOx emissions originate during the quenching process, it is important for the quenching to progress rapidly, thus limiting the time available for NOx formation. It is also important that the fuel and air become intimately intermixed, either in the quench zone itself or in the forwardmost region of the lean burn zone. Otherwise, even though the mixture flowing through the lean burn zone may be stoichiometrically lean overall, it will include localized pockets where the fuel-air ratio is stoichiometrically rich. The fuel rich pockets can arise for a number of reasons, among them injudicious sizing, distribution or density of the combustion air holes in the liners, or local disparities in the supply pressure driving the air jets. These factors can contribute to poor mixing by inhibiting radial penetration of the air jets and/or by limiting the circumferential coverage afforded by the air jets. Because of their elevated fuel-air ratio, the fuel rich pockets will burn hotter than the rest of the mixture, thereby promoting additional NOx formation and generating local xe2x80x9chot spotsxe2x80x9d or xe2x80x9chot streaksxe2x80x9d that can damage the turbine.
It is, therefore, a principal object of the invention to inhibit NOx formation in a gas turbine engine combustor and to do so in a way that produces a favorable temperature profile in the combustion gases flowing from the combustor into the turbine.
According to one aspect of the invention, a gas turbine engine annular combustor includes radially inner and outer liners each having a row of circumferentially distributed combustion air holes penetrating therethrough. The row of holes in the radially outer liner includes large size, major holes and may also include smaller size minor holes circumferentially intermediate neighboring pairs of the major holes. The major holes encourage xe2x80x9cmajorxe2x80x9d jets of combustion air to penetrate radially inwardly a substantial distance toward, and typically beyond, the radial meanline of the combustion chamber annulus. The minor holes, if present, admit xe2x80x9cminorxe2x80x9d jets of combustion air that penetrate radially inwardly a smaller distance than the major jets. The minor jets compensate for any inability of the major jets to spread out laterally (i.e. circumferentially). The holes in the inner liner are circumferentially offset from the major holes of the outer liner. The holes of the inner liner encourage jets of combustion air to penetrate radially outwardly a substantial distance toward, and typically beyond, the radial meanline of the combustion chamber annulus, thus admitting combustion air into sectors of the combustion chamber circumferentially intermediate the major outer jets. This distribution of inner and outer holes ensures a thoroughly blended, regularly distributed fuel-air mixture and thus minimizes NOx formation and contributes to a favorable temperature profile of the combustion gases entering the turbine.
According to a second aspect of the invention, the outer and inner liners each have exactly one row of combustion air holes. Each row includes both large size major holes and smaller size minor holes. In the outer row, the minor holes are circumferentially intermediate the major holes. The major holes of the inner row are circumferentially offset from the major holes of the outer row. The minor inner holes admit air jets of combustion air into the sectors of the combustion chamber circumferentially intermediate the major inner jets and radially inboard of the major outer jets.
According to a third aspect of the invention a gas turbine engine combustor includes radially outer and inner liners each having exactly one row of dilution air holes. The outer row comprises major holes and minor holes circumferentially intermediate neighboring pairs of the major holes. The inner liner has dilution air holes circumferentially offset from the major outer holes. A combustor front end assembly serves as the exclusive vehicle for introducing primary combustion air into the combustion chamber, thus ensuring optimum performance of the combustor""s rich burn zone.
A fourth aspect of the invention is similar to the third aspect, but includes, in the inner liner, smaller size minor inner holes intermediate pairs of the major inner holes.
The principal advantage of the invention is its ability to curtail NOx emissions while presenting a favorable combustion gas temperature profile to the turbine.
The features, operation and advantages of the inventive combustor will become more evident in view of the following description of the best mode for carrying out the invention and the accompanying drawings.